1. Field of the Invention
The present invention relates generally to a Hybrid Electric Vehicle (HEV), and specifically to a method and system to optimize emissions using an adaptive fuel strategy for a hybrid electric vehicle (HEV).
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by Internal Combustion Engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs.
Another alternative solution is to combine a smaller ICE with electric motors into one vehicle. Such vehicles combine the advantages of an ICE vehicle and an electric vehicle and are typically called Hybrid Electric Vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 to Severinsky.
The HEV is described in a variety of configurations. Many HEV patents disclose systems where an operator is required to select between electric and internal combustion operation. In other configurations, the electric motor drives one set of wheels and the ICE drives a different set.
Other, more useful, configurations have developed. For example, a Series Hybrid Electric Vehicle (SHEV) configuration is a vehicle with an engine (most typically an ICE) connected to an electric motor called a generator. The generator, in turn, provides electricity to a battery and another motor, called a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. A Parallel Hybrid Electrical Vehicle (PHEV) configuration has an engine (most typically an ICE) and an electric motor that work together in varying degrees to provide the necessary wheel torque to drive the vehicle. Additionally, in the PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE.
A Parallel/Series Hybrid Electric Vehicle (PSHEV) has characteristics of both PHEV and SHEV configurations and is sometimes referred to as a xe2x80x9cpowersplitxe2x80x9d configuration. In one of several types of PSHEV configurations, the ICE is mechanically coupled to two electric motors in a planetary gear-set transaxle. A first electric motor, the generator, is connected to a sun gear. The ICE is connected to a carrier. A second electric motor, a traction motor, is connected to a ring (output) gear via additional gearing in a transaxle. Engine torque can power the generator to charge the battery. The generator can also contribute to the necessary wheel (output shaft) torque if the system has a one-way clutch. The traction motor is used to contribute wheel torque and to recover braking energy to charge the battery. In this configuration, the generator can selectively provide a reaction torque that may be used to control engine speed. In fact, the engine, generator motor and traction motor can provide a continuous variable transmission (CVT) effect. Further, the HEV presents an opportunity to better control engine idle speed over conventional vehicles by using the generator to control engine speed.
The desirability of combining an ICE with electric motors is clear. There is great potential for reducing vehicle fuel consumption and emissions with no appreciable loss of vehicle performance or drivability. The HEV allows the use of smaller engines, regenerative braking, electric boost, and even operating the vehicle with the engine shut down. Nevertheless, new ways must be developed to optimize the HEV""s potential benefits.
One such area of HEV development is HEV engine operations. In an HEV, the engine has many functions. Its primary function is to provide drive torque. Other functions can include the following: charging the battery, purging a vapor canister, learning the shifts in the fuel delivery system to an adaptive fuel table, powering. an air conditioning (xe2x80x9cA/Cxe2x80x9d) compressor if the compressor is mechanically driven by a front end accessory drive (FEAD) belt, replenishing vacuum to a vacuum reservoir, maintaining catalyst temperature (for optimal emissions), and maintaining engine temperature (for climate control system to provide heat to the passenger compartment). While performing these functions, the HEV engine must optimize emissions and fuel consumption without negatively impacting drivability and performance.
One of the techniques available in an HEV to reduce emissions and fuel consumption is to turn the engine off when it is not needed. If the engine is not running, the electric motor provides the required driving torque.
When running, the engine is used in both drive and vehicle idle conditions. Idle conditions exist when the vehicle is not moving. In an HEV, the engine is generally and ideally off during idle conditions. However, some HEV functions require the engine to remain on even in vehicle idle conditions. One such function can be the maturing of an HEV engine""s adaptive fuel table. Adaptive fuel tables are known in the prior art to optimize emissions from internal combustion engines. See generally, Fuel Controller with an Adaptive Adder, U.S. Pat. No. 5,464,000 to Pursifull, et al. (Ford Motor Company).
As discussed in this referenced patent and known in the prior art, electronic fuel control systems are used predominantly today in most vehicles. The fuel controller systems vary the amount of fuel delivered to the engine cylinders based on the engine speed, mass airflow rate, and the oxygen content of the exhaust. These fuel controllers typically try to maintain the ratio of air and fuel at or near stoichiometry (considered to be approximately 14.6:1 A/F ratio for most types of gasoline) by implementing a closed loop fuel controller. Maintaining A/F at or near stoichiometry allows the catalytic converter to convert the exhaust gas into clean byproducts at an optimal level.
A typical prior art closed loop fuel controller determines the proper amount of fuel to deliver to the engine cylinders as follows. First, the airflow entering the engine is measured and then converted to an estimate of the amount of air charge entering each cylinder. This estimate is then modified by the concentration of oxygen in the exhaust gas (as measured by an exhaust gas oxygen (EGO) sensor). The oxygen content of the exhaust gas directly reflects the A/F ratio of the previous combustion event so that, if the A/F ratio was not near stoichiometry, a correction factor can be applied to the fuel amount delivered for the next combustion event. For example, if the EGO indicates a rich A/F mixture (less then stoichiometry), then the fuel amount will be reduced for the next combustion event. If the EGO indicates a lean A/F mixture, then the fuel amount will be increased for the next combustion event.
The adaptive fuel control feature, as discussed in the referenced patent and known in the prior art, enhances the closed loop fuel controller by learning the long-term xe2x80x9cshiftsxe2x80x9d in the fuel delivery system. The amount of fuel required during closed loop fuel operation varies from engine to engine within a given engine configuration. The variation is due to differences in fuel system components such as fuel injectors and mass airflow sensors, the different degrees to which these components age, and the conditions under which the vehicle is driven. The adaptive fuel controller xe2x80x9clearnsxe2x80x9d these long-term fuel adjustments for the many combinations of engine speed and engine air charge (or airflow) that can occur in the operation of an engine. The adaptive fuel controller learns a fuel shift if the actual A/P ratio is outside of a calibratable range relative to stoichiometry. The amount of the adjustment learned is proportional to how far from stoichiometry the actual A/F was and how quickly the gains used for adaptive corrections are calibrated. These learned or xe2x80x9cadaptedxe2x80x9d adjustments in A/F are then stored in an adaptive fuel table for future use by the closed loop fuel controller when those same engine speed and air charge conditions are encountered again. Once the actual A/F returns to stoichiometry, the adaptive fuel cell is considered to be xe2x80x9cmaturexe2x80x9d.
The adaptive fuel table is a KAM (keep alive memory) type table. There are many different types of adaptive fuel tables. One type uses a matrix and a number of columns for engine speed on the x-axis and a number of rows for airflow on the y-axis. Another method uses engine xe2x80x9cloadxe2x80x9d instead of airflow. Load is a normalized engine air charge defined as the current amount of air charge inducted into the cylinder divided by the maximum amount of air charge possible at that given engine speed. Yet another method uses the airflow dimension only and disregards the effect of engine speed. Regardless of the adaptive fuel table used, the result is the same. When the system is xe2x80x9cadaptingxe2x80x9d to a particular airflow cell, the cell is updated with the air/fuel shift amount. That amount is used the next time the system is at that airflow point.
According to the Ford Motor Company prior art engine calibration guidelines, a vehicle will likely produce more repeatable low emission amounts and meet federal emissions standards if the air and fuel system shifts are learned or adapted before an official FTP (Federal Test Procedure) emissions test. Since only one preparatory (xe2x80x9cprepxe2x80x9d) cycle is allowed before the FTP the system must learn all the air and fuel system shifts in its adaptive fuel tables during the one prep cycle.
Adaptive fuel table strategies in the prior art operate while the engine is running because they need to spend a period of time at a given airflow condition in order to adapt the fuel shift to the appropriate fuel cell in the table until that cell is xe2x80x9cmaturexe2x80x9d. Adaptive fuel strategies typically do not run while the vapor canister is being purged or while some on-board diagnostic monitors are running. Therefore, adaptive fuel, purge, and monitoring strategies tend to compete for engine running time to accomplish their tasks. This situation is exacerbated in an HEV because the vehicle""s ICE is not always on. Therefore, the HEV""s ability to quickly mature its adaptive fuel table is diminished. A new method and system for an HEV to quickly learn the adaptive fuel table must be developed.
Accordingly, an object of the present invention is to provide a method of maturing the adaptive fuel table within one FTP (Federal Test Procedure) emissions test preparatory cycle for a hybrid electric vehicle (HEV).
It is a further object of the present invention to provide a method of maturing the adaptive fuel table within one FTP (Federal Test Procedure) emissions test preparatory cycle for a hybrid electric vehicle (HEV).